U.S. Pat. No. 3,926,602 describes the production of glass-ceramic articles from glasses having compositions within the Li.sub.2 O-FeO-Al.sub.2 O.sub.3 -SiO.sub.2 quaternary nucleated with TiO.sub.2 or a combination of TiO.sub.2 and ZrO.sub.2. The glasses were heat treated at about 850.degree.-1025.degree. C. to cause the development of beta-quartz solid solution as the predominant crystal phase in the interior of the glass-ceramic. Where a temperature of at least 1050.degree. C. was employed, the predominant crystal phase in the interior became beta-spodumene solid solution. Simultaneously with the development of beta-quartz solid solution or beta-spodumene solid solution crystals in the interior of the so-formed glass-ceramic article, a thin, integral surface layer was produced in situ consisting essentially hematite crystals. The surface layer exhibited a metallic luster and, when produced upon previously polished surfaces, would demonstrate a mirror-like finish.
U.S. application Ser. No. 757,582, filed concurrently herewith by the present applicants, discloses the preparation of glass-ceramic articles from glasses having compositions similar to those described in U.S. Pat. No. 3,926,602, supra. Heat treatment of the glasses in an oxidizing atmosphere also produced beta-quartz solid solution or beta-spodumene solid solution as the predominant interior crystal phase, depending upon the heat treatment temperature employed, and a thin, integral surface layer containing hematitie crystals. However, the resulting glass-ceramic article was then thermally treated in a H.sub.2 O-containing reducing atmosphere to convert the hematite crystals to magnetite.
U.S. application Ser. No. 757,585, filed concurrently herewith by the present invention, discloses the production of highly crystalline glass-ceramic articles having an integral surface layer containing metallic iron crystals and wherein either beta-spodumene solid solution or nepheline constitutes the predominant crystal phase in the interior of the articles.
The method of that invention contemplates four basic steps:
First, a batch of the proper composition is melted;
Second, the melt is simultaneously cooled and a glass article of a desired configuration shaped therefrom;
Third, the glass article is heat treated at about 1000.degree.-1300.degree. C. in an oxidizing environment to cause the crystallization in situ of beta-spodumene solid solution or nepheline in the interior portion of the article while, at the same time, effecting crystallization in situ of alpha-hematite (Fe.sub.2 O.sub.3) crystals in an integral thin surface layer; and
Fourth, the crystallized article is exposed to an essentially dry reducing atmosphere at a temperature about 500.degree.-800.degree. C. to reduce the hematite in the surface layer to metallic iron.
Those glass-ceramic articles exhibit such desirable properties as low thermal expansion, i.e., coefficients of thermal expansion over the range of 25.degree.-300.degree. C. of about 0-15 .times. 10.sup.-7 /.degree. C., and high temperature stability. Nevertheless, the production of glass-ceramic articles requires energy to convert glasses to glass-ceramics and, even more importantly, in many applications, e.g., in electrical sealing applications, a coefficient of thermal expansion in the range of about 35-70 .times. 10.sup.-7 /.degree. C. over the temperature interval of 25.degree.-300.degree. C. would be extremely useful.